1. Field of the Invention
The present invention relates to a method of manufacturing a seamless tube formed of titanium materials including pure titanium and titanium alloys by the use of the Mannesmann's method.
2. Description of Related Art
The titanium materials are classified into pure titanium and the titanium alloys such as .alpha.-type titanium alloys, and .alpha.+.beta.-type titanium alloys and .beta.-type titanium alloys. The .alpha.-type titanium alloys include Ti-0.15Pd, Ti-0.8Ni-0.3Mo, Ti-5Al-2.5Sn and the like. The .alpha.+.beta.-type titanium alloys include Ti-8Al-1Mo-1V, Ti-3Al-2.5V, Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-4Zr-2Mo and the like such as small amount of platinum group elements added alloys and platinum group elements plus small amount of Ni, Co, W, Mo added alloys. The .beta.-type titanium alloys include Ti-3Al-8V-6Cr-4Mo-4Zr and the like. In the present invention, the titanium materials used as subject materials include the above-mentioned types of titanium alloys. Pure titanium is a titanium material which mainly has impurities such as H, O, N, Fe, its characteristic is changed according to quantity of O and Fe in particular.
These titanium materials are light and highly corrosion-resistant and in particular a seamless tube formed of them has been expected to be used for piping for use in chemical plants, an oil pressure piping for use in aircraft, and an oil field piping.
The seamless tube has been manufactured by the extrusion method or Mannesmann's method. The extrusion method is suitable for manufacturing a tube of materials inferior in workability, but inferior in manufacturing efficiency. On the contrary, Mannesmann's method is superior in manufacturing efficiency, but requires materials superior in workability.
Some trials for manufacturing a tube formed of a titanium material by Mannesmann's method have been performed, but a manufacturing technique for obtaining a good tube in quality has not been established yet (Reference: "Titanium And Titanium Alloys" volume 1, 1982, P. 313). Because of such circumstances, the seamless tube formed of titanium materials, as provided by JIS H-4630 and the like, has been manufactured by the use of the extrusion method with a little sacrifice of manufacturing efficiency and yield.
In order to manufacture the seamless tube efficiently and economically in general, it is preferable to adopt a continuous seamless tube-manufacturing line including an inclined roll piercing rolling-mill represented by a Mannesmann's piercer. In this case, the seamless tube is manufactured in the following order.
At first, a heated solid billet is subjected to the piercing in the inclined roll piercing rolling-mill or the press-roll type piercer to obtain a hollow piece. The obtained hollow piece is successively subjected to elongating in a mandrel mill or a plug mill to obtain a hollow shell. In the case where the mandrel mill is used in the elongating, the hollow shell is reheated in case of need and then subjected to reducing in a reducer mill while in the case where the elongating is executed in the plug mill, the hollow shell is reheated in case of need and then subjected to sizing in a sizer mill. The hollow piece after piercing is in case of need subjected to second piercing or hollow piece sizing and then subjected to the elongating.
In such a case where the seamless tube formed of the titanium materials is manufactured by the Mannesmann's method, besides the inferiority in workability, the following various kinds of problems have occurred.
If the deformability of the solid billet is insufficient when it is subjected to the piercing, surface defects, such as skin eruptions, flaws and cracks, are produced on an inner surface of the pierced material. In addition, even though a solid billet having a superior deformability is used, if the piercing conditions are not suitable, similar surface defects are produced. Since the titanium materials are inferior in deformability, such surface defects are frequently produced with ease.
Since titanium oxides formed on the surface of the titanium materials prior to the rolling are remarkably difficult to be separated, these titanium oxides are not broken but deposited in a concave portion of the rolls when the titanium materials are subjected to the piercing or the second piercing. Since such titanium oxides stuck to the roll surface are highly lubricious, a problem occurs in that the titanium materials slip and become inferior in intermeshing to the roll during the rolling process.
Furthermore, since the reducing or the sizing is the final process, the result of this reducing or sizing determines the surface quality of a product. Since titanium is originally inferior in hot workability (deformability), if it is intended to secure the dimensional accuracy required in this reducing or sizing, there is a possibility that the surface quality of the product is deteriorated. In particular, in using the reducer mill, it is necessary to contrive a design of roll grooves.
Because of such a state of matters, an art of manufacturing a seamless tube formed of titanium materials by the use of the Mannesmann's method has not been established yet.